28 to 28.5 " - 7/8 size cello
29-30 inches and above - 4/4 size cello
17.75" - 20” - 1/8 size cello
20" - 23” - 1/4 size cello
23”- 26” - 1/2 size cello
26” - 27.5” - 3/4 size cello
27.5" - 28.5 - 7/8 size cello
29” and above - 4/4 size cello
Cellos come in different 'patterns' too, which are often based upon famous cello makers from the past. Some of these patterns are bigger, wider, taller, or thinner than others, and this should also be taken into consideration if the adult student is small of stature. If your adult or young adult is small of size, has short arms or any sort of physical limitation that might make playing a full size cello challenging, the best thing is to try a couple out over time.
Student's past musical experience - Every beginner deserves to have a quality instrument to learn on. If quality is sacrificed to save money, the musical experience is affected. Poor quality instruments are difficult to keep in tune, often have intonation or sound problems, are made of poor quality materials, such as plywood, which will not hold up over time, etc. For someone new to music, it is a balance to choose an instrument that will play well, but not cost a fortune. On the other hand, if your student is an experienced musician, but is new to the cello, you will want to get them the best instrument that you can afford. They will be sensitive to the sound and playability, even if they have never played a stringed instrument. There is nothing more frustrating than an instrument that will not keep up with the player.
Student's level of interest - Some people are more passionate and more committed to their musical experience than others. Some really young children are some of the most passionate musicians around. Keep your student's passion and commitment in mind while shopping. If you have a really serious student, someone who really loves music, or is completely fascinated by the sound that the cello makes, be sure to take special care to get them a quality instrument that they can grow with. Fully carved, solid wood, serious musical instruments can be found in even the smallest sizes for those little ones who are proteges. Your passionate student will advance more quickly because practice will be fun if they have a quality instrument.
Your budget - Cellos can be purchased anywhere from a few hundred dollars up to hundreds of thousands. But for the most part, beginning cellos of a good quality: solid carved, aged woods start at around $1000, depending on the size, maker, etc. I usually carry beginner outfits (cello, bow and case) in different sizes under my Best for Beginner's category for between $1,000 - $2,000. It may seen like a lot of money, but keep in mind that music is more than just a hobby for people. It helps develop the brain, improves coordination, team work, and math skills, and is something that anyone can do at any age.
A few brands to look at if you are starting out and don't want to break the bank would be Eastman cellos, and the new Vivo cellos, which are both great for beginners, but will not hold an advancing student back either. Check out my 'Linda's Picks' for Best for Beginners and For the Budget-Minded to start your cello shopping experience.